In the oil and gas industry, seismic prospecting techniques commonly are used to aid in the search for and evaluation of subterranean hydrocarbon deposits. A seismic prospecting operation consists of three separate stages: data acquisition, data processing, and data interpretation, and success of the operation depends on satisfactory completion of all three stages.
In the data acquisition stage, a seismic source is used to generate an acoustic impulse known as a “seismic signal” that propagates into the earth and is at least partially reflected by subsurface seismic reflectors (such as, interfaces between underground formations having different acoustic impedances). The reflected signals known as “seismic reflections”) are detected and recorded by an array of seismic receivers located at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes. The seismic energy recorded by each seismic receiver is known as a “seismic data trace.”
During the data processing stage, the raw seismic data traces recorded in the data acquisition stage are refined and enhanced using a variety of procedures that depend on the nature of the geologic structure being investigated and on the characteristics of the raw data traces themselves. In general, the purpose of the data processing stage is to produce an image of the subsurface from the recorded seismic data for use during the data interpretation stage. The image is developed using theoretical and empirical models of the manner in which the seismic signals are transmitted into the earth, attenuated by subsurface strata, and reflected from geologic structures.
The purpose of the data interpretation stage is to determine information about the subsurface geology of the earth from the processed seismic data. The results of the data interpretation stage may be used to determine the general geologic structure of a subsurface region, or to locate potential hydrocarbon reservoirs, or to guide the development of an already discovered reservoir.
At present, results from the data interpretation stage are generally limited to the size and shape of large subsurface structures. The descriptions may, for example, provide an indication of the total volume of hydrocarbons which might be retained in such structures. However, seismic data does not provide direct information about the grain size distribution of reservoir sediments, such as sands. Nor does present technology allow the analyst to be able to determine preferred locations within a structure for drilling wells to efficiently produce the contained hydrocarbons, except in a very broad sense. When an exploration well has been drilled, grain size samples can be taken, but present technology does not allow an analyst to accurately characterize grain size in locations other than in the most immediate region of any such well.
As will be understood from this summary of the data interpretation stage of the seismic prospecting operation, it would be desirable to be able to predict the grain size distribution within a subsurface structure of interest using only seismic data and a minimal number of exploration wells, preferably just one exploration well. Such a capability would facilitate estimation of the hydrocarbon volume in place and potential production rates early in the hydrocarbon exploration and development process. As will be understood to those skilled in the art, hydrocarbon volumes and production rates depend on a variety of factors, including the grain size distribution of the sedimentary deposit that makes up the reservoir in which the hydrocarbons are found. The present invention is a method that may be used for predicting the grain size distribution at one or more locations including every location within a sedimentary deposit without having to drill multiple wells to obtain actual samples of the reservoir sediments.